Field of the Invention
The present invention concerns an iron-based soft magnetic powder for dust core, a method of manufacturing the same, and a dust core obtained by using the iron-based soft magnetic powder.
Description of the Related Art
A magnetic core used in alternating magnetic fields is required to have less iron core low and high magnetic flux density. Further, it is also required to have favorable handlability in the manufacturing step and a sufficient mechanical strength not to be fractured during winding for making coils. In view of the foregoings, a technique of coating iron powder particles with electrically insulating resins has been known in the field of the dust core. In the dust core obtained by using the iron powder particles coated with the electrically insulating resin, eddy current loss is suppressed to decrease the iron core loss, and iron powder particles are adhered to each other by the resin to improve the mechanical strength as well.
On the other hand, since it is effective to form the dust core at a high density for improving the magnetic flux density, it is preferred to decrease the amount of the electrically insulating resin for coating the iron powder particles. Particularly for reducing the hysteresis loss to decrease the iron core loss, it is considered to be effective to release the strain of the dust core by annealing at a high temperature. Then, it has been demanded for the development of an iron powder for dust core capable of efficiently insulating the iron powder particles to each other even with a small content of the electrically insulating resin and capable of maintaining good electric resistance even when subjected to a heat treatment at a high temperature such as annealing.
With the view point described above, a technique of using a highly heat resistant silicone resin has been developed as the electrically insulating resin. Further, for the insulating materials other than the resin, a technique of utilizing a film of a glassy compound obtained from phosphoric acid, etc. as the insulation layer has already been known long since (Japanese Patent No. 2710152: Patent Document 1).
By the way, when compared with a silicone resin which is an organic high molecular material, it has been considered that inorganic insulation films described above are naturally excellent in the thermal stability, but they involve a problem that the insulation is lowered when heat treatment at a high temperature (annealing) is performed.
Then, the present applicant has made a study for solving the problem described above and succeeded in providing a dust core having high magnetic flux density, low iron core loss, and high mechanical strength by forming a phosphate conversion coating film containing specific elements and a silicone resin coating film in this order on the surface of an iron-based soft magnetic powder, which has already been granted (Japanese Patent No. 4044591: Patent Document 2).
However, the requirement for improving the performance of the dust core has been increased further compared with that at the time of filing the Patent Document 2 and high magnetic flux density, low iron core loss, and high mechanical strength have now been demanded more than ever before. Among all, the demand for the mechanical strength is high and it has been demanded for a dust core with improved mechanical strength while maintaining high magnetic flux density and low iron core loss.